The present invention relates to the production of alkali metal catalysts on silica supports, particularly the regeneration of alkali metal depleted catalysts.
Alkali metal catalysts on silica supports are known to be useful in catalysing various chemical processes. For instance, the alkali metal caesium catalyses the aldol condensation of formaldehyde with alkyl esters or acids to produce ethylenically unsaturated esters or acids, in particular with methyl propionate to form methyl methacrylate (MMA) and methacrylic acid (MA). However, in continuous industrial applications, the catalytic metal component and catalytic surface area are slowly depleted over time causing consequential loss of catalyst activity. It would be advantageous, therefore, to be able to regenerate the catalyst.
WO99/52628 discloses caesium doped silica supported catalysts. WO99/52628 teaches that for the catalyst to be most effective the surface area should be maintained. The document goes on to teach the general process of impregnation mentioning various salts. No specific solvent is given for caesium except water.
U.S. Pat. No. 4,990,662 discloses the use of metal salts during the process of impregnation. The impregnation of a support with Rb, Cs, K and Na phosphates in aqueous solution by “incipient wetness” or the “pore filling technique” is described. Caesium carbonate is also used and added during catalyst preparation by an unspecified method. This patent also describes a method of adding caesium to the catalyst as part of the vapourised feed to avoid depletion of the catalyst. This technique has the disadvantage of poor distribution of the caesium on the catalyst and excessive coke formation at the front face of the catalyst bed.
U.S. Pat. No. 6,887,822 (PQ Corporation) describes production of a silica hydrogel supported alkali or alkaline earth metal catalyst by impregnation of the hydrogel with an aqueous alkaline solution of the alkali or alkaline earth metal salt. However, the document also teaches that the silica gel surface area is reduced at alkaline pH and elevated temperatures.
WO2009/003722 teaches the impregnation of the catalytic metal onto inorganic oxide supports such as silica using an aqueous acid bath.